This invention relates to transmission of data in electrical form, and more particularly, to apparatus for processing and utilizing ternary data after transmission over a telephone line or the like.
The so-called TlD digital data transmission system recently went into operation in telephone plants to increase the capacity of large in place single cable and older or small screen cable installations. In this type of system a duobinary code is used to transmit 48 channels of data at a rate of 3.152 megabits per second. The bit time period of the data is thus 0.317 microseconds. As a result of the duobinary code the impairment in transmission quality caused by near end cross talk is reduced, which permits the data carrying capacity to be increased.
In a duobinary code, the data signal has a ternary form. The high and low levels of the data signal represent a first binary value and the intermediate level of the data signal represents a second binary value; the data signal abides by the following encoding rule for determining the high and low levels: it assumes different levels for successive occurrences of the first binary value separated by an odd number of bit time periods and assumes the same level for successive occurrences of the first binary value separated by zero or an even number of bit time periods. The frequency spectrum of the transmitted data signal has maxima at direct current and at the bit rate and a broad null at one half the bit rate. The coupling transformer network at the input to the transmission line has a high pass characteristic that distorts the data signal due to the presence of substantial low frequency energy in the spectrum. As a result, the data signal received at end terminals or repeaters in the system exhibits direct current wander, which must be compensated for.
An article entitled "Quantized Feedback in an Experimental 280-Mb/s Digital Repeater for Coaxial Transmission" by F. D. Waldhauer which appeared in IEEE Transactions on Communications, Volume Com-22 No. 1, January 1974, pages 1 through 5, discloses the use of quantized feedback to compensate for direct current wander. Application of this principle to a duobinary system is disclosed in the conference record of the National Telecommunications Conference, Houston, TX, November-December 1980, Volume 2, Session 39, "TID System Overview." Missing low frequencies are supplied by a filtered signal from the repeater output, which reflects the data or bit pattern over many bit time periods.